Television and video images are typically displayed using a 2:1 interlace system. Resolution can be increased by converting the interlaced image to a 1:1 progressive scan image in which each field is applied to all the display lines instead of to alternate display lines.
A known conversion technique is described in EP-A-0192292. In this, pixels in three consecutive display lines from adjacent first and second 2:1 interlace fields are examined. A pair or pixels P and Q are selected from each of two different lines in the second field. These are two pixels that must closely resemble each other. A further pixel D is taken from a line in the first field and is compared with pixel P and Q to derive a median Y. This value is used to generate a new pixel Y in the second field. Use of this technique can lead to aliasing effects being generated in the thus derived progressive scan image.
An improvement is described in U.S. Pat. No. 5,001,563. In this, the median Y is compared with D. If the two differ by more than a predetermined threshold then the new pixel Y is formed from an average of P and Q. This can lead to artefacts being generated in images where two or more pairs of pixels have a high degree of resemblance.
A further technique is described in U.S. Pat. No. 5,532,751. In this, variations between pixels in an image are evaluated to detect edges or contours. If the variation between pixels is below a threshold, the orientation of an edge is deemed to have been successfully estimated and a new pixel is formed from the average of the pixels lying around the estimated orientation of the edge. If the estimate of edge orientation is unsuccessful, a new pixel is formed from the average of two vertically aligned pixels. This technique generates artefacts in pictures with two or more pairs of pixels with high mutual resemblance.
A further improvement of the technique is disclosed in U.S. Pat. No. 6,133,957. In this, the variation between pixels or sets of pixels are computed to reconstruct borders. Two variations are chosen from the ones with smallest value and a pixel is generated as a weighted average of the pixels used to derive the variations.
This technique generates objectionable artefacts in very detailed scenes, when the reconstructed pixel may not match the pixels above and below.